The combination of certain organic chemicals and visible light produces a cytotoxic effect probably through the formation of single oxygen. Hematoporphyrin derivative (HPD) is the most thoroughly studied compound producing this "photodynamic effect". Exposure to shorter wavelengths of light than that associated with cytotoxicity causes HPD to fluoresce. HPD is retained in neoplastic tissue to a greater extent than normal tissue following systemic administration. Because of this property the photodynamic effect can be limited to neoplastic lesions with sparing of surrounding normal tissue. This selective cytotoxicity has been demonstrated in several types of animal and human tumors but has not been reported in intraepithelial neoplasia. Photo-radiation of this malignant lesion would not be affected by the main limitation of this type of therapy, poor tissue penetration of light. Since these lesions are typically less than 4 mm in depth scatter and loss of light energy is not a significant factor. An animal model of intraepithelial neoplasia has been developed by induction with urethane and tetradecanoyl-phorbal-acetate (TPA) in the skin of BDF1 mice. These animals will be given HPD and exposed to light of 630 nm wavelength from an argon ion pumped dye laser. Selective photodestruction of dysplastic lesions will be studied histologically. Information collected from animal studies will be used in designing a pilot human study. It is hoped that this easily administered, essentially non-toxic form of therapy can eventually be applied to pre-malignant lesions of the vulva, vagina, and cervix in humans. Photoradiation of intraepithelial neoplasia in these locations would be preferable to currently used surgical modalities.